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Clean the exposed silicon and grow a thin, high-quality SiO2 gate dielectric by controlled dry thermal oxidation. Deposit polycrystalline silicon, conventionally by low-pressure chemical vapour deposition from silane, dope it in situ or by subsequent implantation and activation, and pattern/etch it to define the two gates. Phosphorus or arsenic gives n+ polysilicon; boron gives p+ polysilicon where the selected gate-work-function process requires it. A traditional single-poly process may use a common gate doping instead; gate and source/drain doping need not be identical.

The patterned gates subsequently serve as self-alignment masks for donor-doped nMOS source/drain and acceptor-doped pMOS source/drain regions. Annealing activates these dopants. Keep the oxide thin and uniform enough for strong electrostatic control but free from pinholes and excessive leakage. A high-permittivity dielectric deposited by atomic layer deposition with a metal gate is a modern alternative, rather than the conventional polysilicon/SiO2 stack drawn.

In physical vapor deposition, material is physically released from a source and transported to the substrate, where it condenses. Three examples are thermal evaporative deposition, electron-beam evaporation and sputtering. Their angular distributions, source heating and energetic bombardment differ, even though all belong to the physical-deposition family.

In chemical vapor deposition, volatile reactants undergo chemical reactions that produce a solid film at or near the substrate, with gaseous products removed. Examples are atmospheric-pressure thermal CVD, low-pressure CVD and plasma-enhanced chemical vapor deposition. These are three established process variants, distinguished by pressure and activation method. Surface chemistry, precursor delivery and temperature determine growth, rather than condensation alone.

Atomic layer deposition is a related cyclic surface-reaction method, but need not be counted as a separate conventional CVD example to satisfy the requested three. Reactive sputtering can involve chemistry, so the boundary is best explained by the dominant source and transport mechanism rather than by claiming that a PVD chamber can contain no chemical reactions.

In atomic layer deposition, expose the surface to a molybdenum precursor, purge unreacted precursor and volatile products, expose it to a sulfur reactant, and purge again. Each half-reaction should saturate the available reactive sites rather than continue indefinitely with dose. Repetition builds a molybdenum disulfide film, with nucleation, incubation and layer closure checked independently.

Self-limited surface chemistry is not the same as depositing exactly one complete crystal monolayer per cycle. A cycle may add only a fraction of a layer, and initially isolated islands can thicken before all bare substrate is covered. Reactions on hydroxylated silicon dioxide can also differ from those on the comparatively inert MoS${}_2$ basal surface. A precursor pair must actually show saturation under the chosen conditions; simply alternating gases is insufficient evidence of ALD.

In metalorganic chemical vapor deposition, metalorganic and sulfur-containing precursors are typically supplied simultaneously or with overlapping residence times, and film growth depends on decomposition, transport and surface reaction rates. Both methods involve chemistry, but ALD separates saturating half-reactions and purge steps. An ALD process need not use a metalorganic precursor, and pulsed CVD is not automatically ALD.

An atomic layer deposition recipe depends on the surface chemistry during the first cycles. A silicon wafer normally carries native oxide with reactive hydroxyl groups. A pristine graphene basal plane has no comparable dense population of dangling bonds or hydroxyl groups, so precursor adsorption and initial nucleation are much less uniform.

Consequently the nominal number of cycles calibrated to give $2\mathrm{nm}$ on silicon can instead give isolated alumina islands, preferential decoration of edges, grain boundaries and defects, and uncovered graphene between islands. Adsorbed transfer residues may provide additional, irregular nucleation sites. Once alumina has nucleated, later cycles grow on the alumina normally; the problem is delayed, spatially uneven nucleation. The resulting local island height can exceed the film's area-averaged equivalent thickness.^[ref]

Two suitable improvements are:

Ozone or a mild plasma is another possible activation route, but it must be controlled because creating covalent defects can degrade the electrical properties that motivate using graphene.